As described in copending patent application No. 587,929 filed Mar. 9, 1984, a standard manipulator has a stationary base on which a carousel is rotatable about a vertical axis by means of a motor mounted on the base. A main support arm is pivotal on the carousel about an inner horizontal axis perpendicularly intersecting the vertical axis by means of a motor mounted on the carousel. A counterweight is provided for counterbalancing the offcenter weight of the main arm and the structure carried by it.
The upper end of the main arm carries an outrigger arm rotatable on the main arm about an outer horizontal axis by means of a motor carried on the main arm. The outer end of this outrigger arm carries a so-called mechanical hand comprising a head rotatable about an inner head axis perpendicularly intersecting the outer outrigger-arm axis and carrying in turn a tool holder rotatable on the head about an intermediate holder axis perpendicularly intersecting the head axis. This holder in turn can rotate a tool about an outer tool axis perpendicular to the holder axis. Drives are provided at the rear end of the outrigger arm for rotation about the various axes.
Thus, and as also described in German patent document No. 2,435,156 filed July 22, 1974 by Hans Richter, such an arrangement has six different pivot axes--the vertical one between the base and carousel, the inner horizontal one between the inner arm end and the carousel, the outer horizontal one between the outer arm end and the outrigger, and the head, holder, and tool axes--so that the tool can assume virtually any necessary angle of attack relative to the workpiece. The three-shaft drive for the tool head, as described in German patent document No. 2,402,829 filed by T. Ono et al based on a Japanese application filed Jan. 25, 1973, uses three concentric drive shafts for the inner, intermediate, and outer head-assembly axes to ape the motions of a hand and wrist.
As further described in German patent document No. 2,619,336 filed by P. Pardo et al with a claim to a French priority of May 15, 1975, such arrangements have the disadvantage that, for instance, to bring the tool into a position at a right angle to the inner head it is necessary first to rotate the intermediate and inner parts, that is the head and holder, and then the intermediate and outer parts, that is the holder and tool, to achieve the desired orientation. It is therefore impossible to move the tool smoothly along a continuous arc.
Accordingly U.S. Pat. No. 4,068,536 issued Jan. 17, 1978 and German patent document No. 2,927,485 filed July 7, 1979 by K. Wilfinger et al propose using nonperpendicular head-end axes, so that the inner and intermediate parts meet at an oblique plane. Thus the intermediate shaft extends at an angle to the parts to both sides of itself. All of the axes intersect at a point so that the outer tool axis is always perpendicular to a point on a spherical surface. Thus the tool can trace a smooth arc.
In all of these systems the outer pivot axis can be aligned with the axis on which the coaxial drive shafts are centered, so that in this position the program that operates the manipulator must decide whether to effect rotation of the tool about this axis either by rotation of just the tool, or of the tool and holder. This ambiguity can only be resolved by programming routines that complicate the system while conferring no particular benefit.